Men are pictured next to damaged buildings at a site hit yesterday by airstrikes in the rebel held al-Shaar neighbourhood of Aleppo, Syria November 17, 2016. REUTERS/Abdalrhman Ismail

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By Lisa Barrington

BEIRUT (Reuters) - Air strikes and shelling killed at least 25 people in rebel-held eastern Aleppo on Thursday on a third day of renewed bombing, according to a group that monitors Syria's civil war, and the mayor of the besieged sector warned of an acute lack of fuel and food as winter encroaches.

The bombardment of eastern Aleppo restarted on Tuesday after a four-week pause, part of a wider military escalation by the Syrian government and its allies, including Russia, against insurgents.

The battle for Syria's second city marks a crucial phase of the five-year civil war, in which hundreds of thousands of people have been killed, and its capture would be a major triumph for President Bashar al-Assad.

His government said on Tuesday it was striking what it called "terrorist strongholds" in Aleppo.

Health officials in the opposition-held part of Aleppo said they had recorded 45 deaths and admitted 363 wounded people as a result of air strikes up to and including Wednesday this week.

Russia says it is not bombing the city, but it is using an aircraft carrier, missiles fired from another warship, and planes launched from air bases in Russia and Syria to strike targets around the country.

The siege and intense bombardment of east Aleppo have created a dire humanitarian crisis, aggravated by frequent air strikes on hospitals and the disruption and pollution of water supplies. Medicines, food and fuel are severely depleted.

"There is only enough to keep the bakeries going to give people at least some bread. People are only getting about 15 percent of what they need," Brita Hagi Hassan, president of the city council for opposition-held Aleppo, told Reuters.

The United Nations says 250,000 civilians remain in Aleppo's opposition-controlled neighborhoods, effectively under siege since the army, aided by Iranian-backed militias and Russian jets, cut off the last road into rebel districts in early July

International charity Oxfam said it had moved a large electricity generator to the Suleiman al-Halabi water station that is located on the frontline between east and west Aleppo and still serves both sides of the city under an agreement.

It said all other aid to the besieged area remains cut off.

The Syrian Observatory for Human Rights, a Britain-based organization that monitors the war, said shelling and air strikes from helicopters and jets hit the eastern half of the city, causing severe damage. Air strikes also hit rebel-held areas west and south of Aleppo.

Shelling of government-held western Aleppo by rebels during a failed counter-attack they staged earlier this month killed dozens of people, the United Nations said.

A senior official in the military alliance fighting in support of Assad told Reuters last week the plan was to recapture the city before a new U.S. president takes office in January.

Donald Trump's surprise election last week raised hopes in Damascus and Moscow that the United States - which has been providing support for rebels - might change its Syria policy.

Russian news agencies on Thursday cited deputy foreign minister Mikhail Bogdanov as saying Russia had begun talking to Trump's team about Syria and hoped his administration would take a new approach to the Syrian crisis.

(Reporting by Lisa Barrington in Beirut. Additional reporting by Vladimir Soldatkin in Moscow and Idrees Ali in Washington; Editing by Angus McDowall and Mark Trevelyan)